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The iPhone and iPad maker, along with HarperCollins, Hachette Livre, Verlagsgruppe Georg von Holtzbrinck-owned Macmillan, and CBS-owned Simon & Schuster (ZDNet is also owned by CBS), all agreed to legally binding conditions that would ease pricing restrictions on Amazon and other e-book sellers.

A fifth publisher, Penguin -- owned by U.K. group Pearson -- is still under investigation as the publisher "chose not to offer commitments," but the Commission said it was still in discussions with the publisher.

The European Commission said in a statement that the companies, "may have contrived to limit retail price competition for e-books in the European Economic Area (EEA), in breach of EU antitrust rules." In order to address these concerns, the e-book publishers have offered to, "terminate on-going agency agreements and to exclude certain clauses in their agency agreements during the next five years."

The Commission said it will open an antitrust case to investigate whether the publishers were "helped" by Apple to fix e-book pricing. It is also alleged that the actions could have blocked rivals, such as Amazon, which has a different 'wholesale' pricing model, and ultimately hurt consumers.

Amazon's pricing model gives much greater flexibility to e-book makers and sellers, allowing them to price what they like for their work, and even at a loss, while Apple takes an 'agency' fee of 30 percent of each e-book sale.

The publishers also agreed to end a "most-favored-nation" clause, which allows a retailer to apply a lower retail price for an e-book by another retailer, regardless of the model used by other retailers. This meant that retailers could take a slice of the profits, which could have a knock-on effect to smaller booksellers.

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The Commission said in the statement that it was concerned that the switch to these agency contracts, "may have been coordinated between the publishers and Apple, as part of a common strategy aimed at raising retail prices for e-books or preventing the introduction of lower retail prices for e-books on a global scale."

The Commission said it was "satisfied" that these commitments will "remedy the identified competition" that Europe's executive body identified.

EU antitrust chief Joaquin Almunia said: "While each separate publisher and each retailer of e-books are free to choose the type of business relationship they prefer, any form of collusion to restrict or eliminate competition is simply unacceptable."

He added: "The commitments proposed by Apple and the four publishers will restore normal competitive conditions in this new and fast-moving market, to the benefit of the buyers and readers of e-books."